(1) Field of the Invention
The present invention relates to an unbalance point positioning method and an apparatus therefor which are used for measuring positions and amounts of unbalanced and correcting and shaping unbalance points of rotatable objects such as a rotor of an electric motor, a fan, a rotary shaft and the like which require a good rotational balance.
(2) Description of the Prior Art
In conventional no-mark, no-sensor type positioning methods and apparatus, a measured object is stopped by utilizing an unbalance phase signal as a trigger for inputting a pulse signal to a preset counter, and selecting a preset value for stopping an unbalance point at an appropriate position. The above pulse signal may comprise a stepping motor drive pulse signal or a pulse signal output from a rotary encoder or the like representing every unit angle of rotation. In this case, the measured object is maintained in constant low-rate rotation for a time after start of deceleration from high-rate rotation for measurement and before stoppage, to wait for the preset counter to finish counting. The measured object is stopped as soon as the count of the pulse signal for every unit angle of rotation reaches the preset value.
One example of unbalance signal detecting device used in conventional unbalance measuring apparatus is shown in FIG. 8.
In FIG. 8, a pressure sensor 115 detects vibrations resulting from revolution of an unbalance point of a measured object such as a rotor 116 and transmitted through a bearing 117 and a horizontal member 118.
The vibrations detected by the pressure sensor 115 include not only the vibrations due to the revolution of the unbalance point but horizontal vibration components due to vibrations of the ground and other sources.
The frequency of vibrations due to the revolution of the unbalance point equals the rotational frequency of the rotor 116. The resulting signal may therefore be passed through a bandpass filter which allows passage of a frequency synchronized with the rotational frequency, to secure an unbalance signal stripped of the horizontal vibration components due to vibrations of the ground or the like.
The known detecting device includes an amplifier for compensating for attenuation of the signal occurring with the passage through the bandpass filter.
Further, a measured object support mechanism used in the known unbalance measuring apparatus, as shown in FIG. 8, has the bearing 117 rotatably supporting a shaft of the measured object 116. The bearing 117 is secured to the oscillatable horizontal member 118.
The oscillatable member 118 is supported at opposite ends thereof by plate springs A to be oscillatable right and left. The vibration sensor 115 is supported as sandwiched between one of the plate springs A and a base block B.
The plate springs A are urged toward the vibration sensor 115 to hold the vibration sensor 115 against downward slipping.
In the known positioning method, however, slips occur between a drive belt and the measured object when the measured object in constant low-rate rotation is instantly stopped when the counter finishes counting. Such slips are the main cause of positioning errors.
Moreover, it is necessary to maintain the low-rate rotation for a certain length of time to wait for the completion of counting. This prevents shortening of the time taken for a positioning operation.
In order to minimize the slips occurring between the measured object and drive belt at positioning times, two troublesome operations are required, i.e. adjustment of drive motor slow-down time and adjustment of the preset value of the preset counter for stopping the unbalance point at a selected position. These two adjustment operations interfere with each other, and are therefore difficult to carry out smoothly.
Further, the known method has a further disadvantage of processing complexity in that it requires speed setting and control signals for the high-rate rotation at times of unbalance measurement and for the constant low-rate rotation at times of unbalance point positioning.
There is a problem of high cost since the positioning apparatus must include, as essential components thereof, a preset counter and a device for generating a pulse signal every unit angle of rotation.
The known unbalance signal detecting device used in the unbalance measuring apparatus is capable of removing horizontal vibration components due to vibrations of the ground or the like by passing the signal through a bandpass filter. However, an amplifier is needed to compensate for the signal attenuation resulting from the passage through the bandpass filter. This results in a complicated and expensive detecting device. In addition, the detecting precision of the device cannot be improved since it is impossible to remove the external vibrations synchronized with the rotation of the measured object.
Furthermore, whenever the rotational frequency of the measured object is varied at times of unbalance measurement, the frequency passed through the bandpass filter must be varied or an expensive filter such as a tracking filter is required.
The known measured object support mechanism used in the unbalance measuring apparatus does not allow high precision measurement since slight vibration energy is absorbed by the plate springs supporting the oscillatable member.
There is the further problem that resonance of the plate springs and associated elements and distortion of the plate springs per se obstruct accurate unbalance measurement.